Temperature compensation circuit and a variable gain amplification circuit

ABSTRACT

A temperature compensation circuit comprises a signal source to output a first signal corresponding to a temperature change of an ambient temperature to a predetermined temperature, and a multiplier to multiply an external gain control signal and the first signal and output a second signal proportional to the temperature change and the first signal to a variable gain amplifier to perform the temperature compensation with respect to the variable gain amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-337175, filed Nov.6, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a temperature compensationcircuit performing a temperature compensation with respect to a gaincharacteristic of a variable gain amplifier fabricated by MOStransistors and gain-controlled by an external gain control signal, anda variable gain amplification circuit using the same temperaturecompensation circuit.

[0004] 2. Description of the Related Art

[0005] In a conventional art, a variable gain amplifier has beendeveloped so that a gain varies with an exponential function in respectto a gain control signal Vc. For example, U.S. Pat. No. 6,215,989 B1,the entire contents of which are incorporated herein by reference, avariable gain amplifier fabricated by bipolar transistors having aninput/output characteristic representing an exponential function.

[0006] In late years, a variable gain amplifier using MOS transistorswhich is beneficial to cost reduction is developed. However, since theMOS transistor has an input/output characteristic of square-lawcharacteristic, a circuit must be improved to obtain an exponentialfunction characteristic.

[0007] The applicant provides a technique of realizing a variable gainamplifier utilizing a fact that the MOS transistor operating in a weakinversion region has an exponential-law characteristic expressed by anexpression (4) (refer to the U.S. patent application Ser. No.09/950,630, the entire contents of which are incorporated herein byreference).

[0008] However, since a thermal voltage W (=k/T/q) of the equation (4)is a variable proportional to the temperature T, a gain varies by thetemperature as shown in FIG. 10. Therefore, this temperature variationmust be suppressed.

BRIEF SUMMARY OF THE INVENTION

[0009] It is an object of the invention to suppress a temperaturevariation of a gain characteristic of a variable gain amplifier using aMOS transistor.

[0010] According to an aspect of the present invention, there isprovided a temperature compensation circuit performing a temperaturecompensation with respect to a gain characteristic of a variable gainamplifier fabricated by MOS transistors and gain-controlled by anexternal gain control signal, the apparatus comprising: a signal sourceconfigured to output a first signal corresponding to a temperaturechange of an ambient temperature to a predetermined temperature; amultiplier configured to multiply the external gain control signal andthe first signal and output a second signal proportional to thetemperature change and the first signal; and a temperature compensationdevice configured to transfer the second signal to the variable gainamplifier to perform the temperature compensation with respect to thevariable gain amplifier.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0011]FIGS. 1A and 1B show block diagrams of variable gain amplificationcircuits performing a temperature compensation with respect to a gain ofa variable gain amplifier according to an embodiment of the presentinvention;

[0012]FIG. 2 shows a circuit diagram of a current source 101 a having atemperature dependency;

[0013]FIG. 3 is a circuit diagram of a current source 101 b which is amodification of the current source 101 a having a temperaturedependency.

[0014]FIG. 4 shows a circuit diagram of a current source having notemperature dependency;

[0015]FIG. 5 is a circuit diagram of a multiplier used for realizing avariable gain amplifier using MOS transistors and having no temperaturedependency;

[0016]FIG. 6 shows a direct conversion type radio apparatus;

[0017]FIG. 7 shows a block diagram of a variable gain amplifier 102utilizing a weak inversion region of a MOS transistor;

[0018]FIG. 8 shows a concrete circuit diagram of a control signalconverter 702 of FIG. 7 that utilizes a weak inversion region of a MOStransistor;

[0019]FIG. 9 shows a circuit diagram of a control signal converter 702including a concrete circuit of a current source 801 of FIG. 8; and

[0020]FIG. 10 shows a graph to show a temperature characteristic of thegain control without temperature compensation in a variable gainamplifier 102 utilizing a weak inversion region of a MOS transistor ofFIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention realizes a variable gain amplifier circuitusing MOS transistors operable in a weak inversion region. It isgenerally known that an input/output characteristic shifts from asquare-law characteristic to an exponential-law characteristic when adensity of current flowing through a MOS transistor is decreased. Theregion where a MOS transistor has the exponential-law characteristics isreferred to as a weak inversion region.

[0022] In contrast, a region having a square-law characteristic isreferred to as a strong inversion region, and a gate-to-source voltagewhen it is changed from the weak inversion region to the stronginversion region is referred to as a threshold voltage. In other words,at the time of VGS≧VTH (VGS: gate-to-source voltage, VTH: thresholdvoltage), the MOS transistor becomes a strong inversion state, and atthe time of VTH/2<VGS<VTH, the MOS transistor becomes a weak inversionstate. According to this technique, since a characteristic of MOStransistor in itself is used, a circuit does not become complicated, andlow power consumption can be realized.

[0023] In weak inversion region, a I_(D)-V_(GS) characteristic(input/output characteristic) is expressed by $\begin{matrix}{I_{D} = {I_{ON} \cdot {\exp \left( \frac{V_{GS} - V_{ON}}{n \cdot V_{T}} \right)}}} & (1)\end{matrix}$

[0024] where ID is a drain current, I_(ON) is an ON current, and V_(ON)is an ON voltage, and I_(ON), V_(ON) and n are constants determined by aproduction process of a MOS transistor (for example, doping density ofimpurities). VT=(k·T)/q, where VT: thermal voltage, a k: Boltzmannconstant, T: temperature, and q: elementary charge quantity.

[0025] The gain control signal (first gain control signal) Vc can beexponentially gain-controlled when a variable gain amplifier 102 shownin FIGS. 7 and 8 is operated in the weak inversion region. A gaincharacteristic of an output current to an input current is expressed byan expression (2) with respect to the second gain control signal Vy, andthe transfer function of gain control signal converter 702 is expressedby an equation (3). A concrete circuit configuration of a current source801 of FIG. 8 refers to FIG. 9, and a gain control signal current 1 cflowing through a current source 901 of FIG. 9 indicates the currentproportional to a gain control signal Vc. $\begin{matrix}{\frac{I_{out}}{I_{in}} = \frac{1}{1 + {\exp \left( \frac{V_{y}}{n \cdot V_{T}} \right)}}} & (2) \\{V_{y} = {{n \cdot V_{T} \cdot I}{\,_{n}\left\{ {{\exp \left( \frac{b \cdot V_{C}}{n \cdot V_{T}} \right)} - 1} \right\}}}} & (3)\end{matrix}$

[0026] where b indicates a proportional constant. When the equation (3)is inserted in the equation (2), an equation (4) showing an exponentialgain control is obtained. $\begin{matrix}\begin{matrix}{\frac{I_{out}}{I_{in}} = \frac{1}{1 + {\exp \left( \frac{V_{Y}}{n \cdot V_{T}} \right)}}} \\{= {\exp \left( \frac{{- b} \cdot V_{C}}{n \cdot V_{T}} \right)}}\end{matrix} & (4)\end{matrix}$

[0027] However, since a thermal voltage VT(=(k·T)/q) of the equation (4)is a variable proportional to the temperature T, a gain varies by thetemperature as shown in FIG. 10. Therefore, this temperature variationmust be suppressed. The present invention is directed to temperaturecompensation of a gain of the variable gain amplifier circuit.

[0028] There will now be described embodiments according to theinvention in conjunction with drawings.

[0029] If the thermal voltage VT of the equation (4) is represented as athermal voltage VTo with a predetermined temperature To, and atemperature change of an ambient temperature T to a predeterminedtemperature To, for example, a difference is indicated by ΔT, VT isexpressed with VTo (1+ΔT/To), and the equation (4) is converted to anequation (5). $\begin{matrix}{\frac{I_{out}}{I_{in}} = {\exp \left\{ \frac{{- b} \cdot V_{C}}{n \cdot {V_{To}\left( {1 + \frac{\Delta \quad V}{T_{o}}} \right)}} \right\}}} & (5)\end{matrix}$

[0030] If a temperature dependency is utilized in order to convert Vcinto Vc (1+ΔT/To), the equation (5) is converted to an equation (6).$\begin{matrix}\begin{matrix}{\frac{I_{out}}{I_{in}} = {\exp \left\{ \frac{{- b} \cdot {V_{C}\left( {1 + \frac{\Delta \quad V}{T_{o}}} \right)}}{n \cdot {V_{To}\left( {1 + \frac{\Delta \quad V}{T_{o}}} \right)}} \right\}}} \\{= {\exp \left( \frac{{- b} \cdot V_{C}}{n \cdot V_{To}} \right)}}\end{matrix} & (6)\end{matrix}$

[0031] The equation (6) is the equation having no temperaturedependency. It can be understood to convert a gain control signal Vcinto a signal represented by Vc (1+ΔT/To) to avoid a temperaturedependency of a gain characteristic of an output current I_(out) to aninput current I_(in).

[0032] (First Embodiment)

[0033]FIG. 1A shows a block diagram expressing a first basic concept ofa variable gain amplification circuit performing a temperaturecompensation with respect to a gain of a variable gain amplifieraccording to an embodiment of the present invention. In order togenerate a gain control signal current 1 c (T)=Ic(ΔT)=g1*Vc(1+ΔT/To)(g1: conductance (A/V)) to be given to the variable gain amplifier (VGA)102, the temperature variable (1+ΔT/To) of the output signal ofthe-current source having a temperature dependency is multiplied withthe gain control signal current Ic.

[0034] In the present invention, the output current 1 c(ΔT) as themultiplication result is used as a gain control signal of the variablegain amplifier. A gain control signal input terminal 103 is a terminalto receive a gain control signal Vc supplied from the outside. Anamplifier 105 has a function as a voltage-current converter thatconverts the gain control signal Vc to the gain control signal current 1c in addition to a function as an amplifier.

[0035]FIG. 1B shows a block diagram illustrating a second basic conceptof a variable gain amplification circuit performing a temperaturecompensation with respect to a gain of a variable gain amplifieraccording to an embodiment of the present invention. FIG. 1B differsfrom FIG. 1A in generation of an output signal by adding a gain controlsignal I_(CNTO) (=g2*Vc=c*Ic) (g2: conductance (A/V), c: coefficient)which substantially does not depend upon a temperature and a gaincontrol signal I_(CNT1) depending upon a temperature (T). In this case,the signal I_(CNT1(T)) makes it possible to change a proportionalcoefficient of temperature into a desired value from 1/To. The gaincontrol signal I_(CNTO) that does not depend upon a temperaturesubstantially can be generated by a conventional differential amplifier105 b using a source degenerate resistor.

[0036] The current source used for making the gain control signal Vchave a temperature dependency is explained hereinafter. FIG. 2 is acircuit diagram showing a concrete circuit 101 a of the current source101 having a temperature dependency as shown in FIGS. 1A and 1B. Areference “M” shows a MOS (Metal Oxide Semiconductor) transistor, and asubscript “P” indicates a P type and a subscript “N” indicates an Ntype. The circuit of current source 101 a is known broadly as a Widlarcurrent source. (reference: P. R. Gray and R. G. Meyer, “Analysis andDesign of ANALOG INTEGRATED CIRCUITS, 3rd edition”, WILEY.) However, theconventional Widlar current source is fabricated by bipolar transistors.

[0037] In the current source 101 a related to the embodiment, MOStransistors are used for integrating the current source, and MOStransistors MN 30 and MN31 are operated in weak inversion regions tooutput a current with a temperature dependency. In other words, thecurrent source 101 a comprises a MOS transistor MN30 having a sourceterminal connected to a ground terminal and a MOS transistor MN31 havinga source terminal connected to the ground terminal via a resistor RT.The gate terminal of the MOS transistor MN30 is connected to the gateand drain terminals of the MOS transistor MN31. Substantial equalcurrents flow through the drain terminals of the MOS transistor MN30 andMN31. The MOS transistors MN30 and MN31 are operated in the weakinversion region.

[0038] The circuit configuration of the current source 101 a will bedescribed hereinafter. The transistors MN30 and MN31 operate as acurrent mirror circuit, and the gate terminal and drain terminal of thetransistor MP31 construct the input terminal of the current mirrorcircuit. The drain terminal of the transistor MN30 is connected to theoutput terminal of the current mirror circuit, the source terminalthereof is grounded, and the gate terminal thereof is connected to thegate and drain terminals of the transistor MN31. The source terminal ofthe transistor MN31 is grounded via a resistor RT. The drain terminal ofthe transistor MN32 is connected to the input terminal of the currentmirror circuit, and the source terminal thereof is connected to thedrain terminal of the transistor MN31, and the gate terminal thereof isconnected to the drain terminal of the transistor MN30 and a start upcircuit 200. This current source 101 a has two stable points of acurrent value, and the start up circuit 200 is provided for making thecurrent source active to obtain a predetermined current value. The W/Lratio (W: gate width, L: gate length) of the transistors MN30 and MN31is set at 1:M. The transistors MN30 and MN31 are designed to increase Wfor the purpose of reducing a current density so that they operate inthe weak inversion region.

[0039] The operation of the current source 101 a is explainedhereinafter. The input/output characteristic of the transistors MN30 andMN31 is expressed with the equation (1) to operate the transistor MN30and MN31 in the weak inversion region. Therefore, the voltage VR appliedto both ends of the resistor RT is expressed with the followingequations. $\begin{matrix}\begin{matrix}{V_{R} = {V_{{GS},{MN30}} - V_{{GS},{MN31}}}} \\{= {{n \cdot V_{T} \cdot \ln}\quad M}}\end{matrix} & (7)\end{matrix}$

[0040] Therefore, the operating current I_(out) (T) is expressed with anequation (8). $\begin{matrix}\begin{matrix}{{I_{out}(T)} = \frac{V_{R}}{R_{T}}} \\{= \frac{{n \cdot V_{T} \cdot \ln}\quad M}{R_{T}}} \\{= {\left( \frac{{n \cdot \ln}\quad M}{R_{T}} \right){V_{To}\left( {1 + \frac{\Delta \quad T}{T_{o}}} \right)}}} \\{= {I_{o}\left( {1 + \frac{\Delta \quad T}{T_{o}}} \right)}}\end{matrix} & (8)\end{matrix}$

[0041] where Io=(n*1nM/RT) VTo. Therefore, it is understood that thecurrent source 101 a has a temperature dependency.

[0042]FIG. 3 shows a concrete circuit 101 b of the current source 101 ofFIGS. 1A and 1B having a temperature dependency. In FIG. 3, thetemperature coefficient is set to a value different from 1/To. Thecircuit of FIG. 3 differs from that of FIG. 2 in inputting an output ofthe transistor MP32 and an output of a constant current source P*Io (Pindicates an adjustable coefficient) without a temperature dependency toa current mirror circuit fabricated by transistors MN33 and MN34, andextracting an output current I_(out) (T) from transistor MN34 of thecurrent mirror circuit.

[0043] According to the above configuration, the output current Iout (T)is expressed with the following equations. $\begin{matrix}\begin{matrix}{{I_{out}(T)} = {{I_{o}\left( {1 + \frac{\Delta \quad T}{T_{o}}} \right)} - {p \cdot I_{o}}}} \\{= {\left( {1 - p} \right)I_{o}\left\{ {1 + \frac{\Delta \quad T}{T_{o}\left( {1 - p} \right)}} \right\}}}\end{matrix} & (9)\end{matrix}$

[0044] An equation (10) is provided when the current is multiplied by1/(1−p). $\begin{matrix}{{I_{out}(T)} = {I_{o}\left\{ {1 + \frac{\Delta \quad T}{T_{o}\left( {1 - p} \right)}} \right\}}} & (10)\end{matrix}$

[0045] The temperature coefficient can be changed by selecting Pproperly.

[0046]FIG. 4 shows a circuit of a current source having substantially notemperature dependency, and this current source is used as a currentsource 501 of FIG. 5 having no temperature dependency as describedbelow. This circuit uses that the polarity of a temperature dependencyof thermal voltage V_(T) and that of a temperature dependency of thethreshold voltage V_(TH) of a MOS transistor is different. In otherword, results obtained by multiplying desired coefficients (α, β) byrespective current outputs are added by an adder 403. The additionresult is output as the current having substantially no temperaturedependency to an output terminal 404. A circuit 401 corresponds to thecurrent source 101 a of FIG. 2. The current source is proportional tothe thermal voltage V_(T) and has a temperature dependency. A circuit402 comprises a current source proportional to the threshold voltageV_(TH) and having a temperature dependency. In other words, the currentsource comprises a MOS transistor M_(N40) whose source terminal isgrounded, a resistor R_(TH) through which a gate terminal of the MOStransistor M_(N40) is grounded, and a MOS transistor M_(N41) having agate terminal connected to a drain terminal of the MOS transistorM_(N40) and a source grounded via the resistor R_(TH). The drainterminal of the MOS transistor M_(N41) is connected to an outputterminal.

[0047] When the transistor MN40 is operated in the weak inversionregion, V_(GS) can be approximated to the threshold voltage Therefore,an output current of the circuit 402 is expressed by an equation (11).$\begin{matrix}\begin{matrix}{{I_{out}(T)} = \frac{V_{TH}}{R_{TH}}} \\{= \frac{V_{THo}\left( {1 - {{q \cdot \Delta}\quad T}} \right)}{R_{TH}}}\end{matrix} & (11)\end{matrix}$

[0048] where V_(THo) expresses a threshold voltage of predeterminedtemperature To, q expresses a temperature coefficient of a thresholdvoltage and RTH expresses a resistor. The circuit 402 comprises acurrent source having a threshold voltage as a reference, and isreferred to as a threshold reference circuit.

[0049] In this circuit, the current flowing through the transistor MN40is based on a current depended upon the thermal voltage V_(T). However,V_(GS) can be approximated to the threshold voltage if the transistorMN40 is not operated in the weak inversion region as described above.Therefore, no problem occurs even if a bias is set using the currentdepended upon V_(T). Even if the transistor MN40 is used in the weakinversion region, the temperature dependency of the threshold voltagecan be anticipated. Therefore, there is a case that the transistor MN40need not be used always in the weak inversion region.

[0050]FIG. 5 shows a multiplier 104 to generate a current having atemperature dependency and a gain control current 1 c. The multiplier104 comprises current sources 501, 502 and 503 and MOS transistorsM_(p50), M_(p51), M_(p52) and M_(p53). The current source 501 isconnected to a node of drain terminals of the MOS transistors M_(p50),and M_(p51) and the current source 502 is connected to a node of drainterminals of the MOS transistors M_(p52) and M_(p53). The gate of theMOS transistors M_(p51) and M_(p52) is connected to a voltage sourceV_(BB). The gate and source terminals of the MOS transistor M_(p50) andthe gate terminal of the MOS transistor M_(p53) is grounded via thecurrent source 503. The source terminals of the MOS transistors M_(p51)and M_(p52) are grounded. The source terminal of the MOS transistorM_(p53) is connected to the output terminal 504.

[0051] The multiplier 104 makes a gate-to-gate voltage common to twodifferential transistor pairs M_(p50), M_(p51) and M_(p52), M_(p53).Therefore, the ratio between the tail current Io and gain controlcurrent Ic can be substantially equalized to the ratio between the tailcurrent Io (1+ΔT/To) and output current Ic (T). The tail current Io isoutput by a current source 501 and a current having substantially notemperature dependency, and the gain control current Ic is a gaincontrol current before the temperature compensation that is output by acurrent source 503. The tail current Io (1+ΔT/To) is a current with atemperature dependency output from the current source 502, and theoutput current Ic (T) is a current obtained at an output terminal 504.As a result, the output current 1 c (T) is determined by multiplicationof a temperature variable (1+ΔT/To) of the tail current Io (1+ΔT/To)having the temperature dependency with the gain control current 1 cbefore temperature compensation. Then it is expressed this with thefollowing equations. $\begin{matrix}{\frac{I_{c}}{I_{o}} = \frac{I_{c}(T)}{I_{o}\left( {1 + \frac{\Delta \quad T}{T_{o}}} \right)}} & (12) \\{{I_{c}(T)} = {I_{c}\left( {1 + \frac{\Delta \quad T}{T_{o}}} \right)}} & (13)\end{matrix}$

[0052] This equation is an equation when transistors MP50, MN51, MP52and MP53 are operated in the weak inversion region, that is, when thetransistors MP50, MN51, MP52 and MP53 had an exponential input/outputcharacteristic. However, this equation can be applied in approximationeven where the transistor MP50, MN51, MN52 and MP53 had a square-lawinput/output characteristic.

[0053] In the above explanation, the current source with the temperaturecoefficient of 1/To is applied, but it can be substituted for a currentsource with temperature coefficient different from 1/To as shown in FIG.3.

[0054] The above embodiment is described for the temperaturecompensation of FIG. 1A, but is applicable to the temperaturecompensation of FIG. 1B. The multiplication of the gain control currentsignal Vc with the current of the current source 101 having atemperature dependency can be performed by a multiplier 104 shown inFIG. 5. A current source having a temperature dependency uses thecurrent source of FIG. 2 or the current source of FIG. 3. In this case,the control current subjected to a temperature compensation isI_(CNT1(T)) of FIG. 1B. The circuit for gain control current havingsubstantially no temperature dependency may use a conventionaldifferential amplifier 105 b. Therefore, detailed description of thecircuit is omitted.

[0055] In a case of FIG. 1A, the gain control current Ic (T) having atemperature dependency is generated according to the equation (12), andis input to the current source 901 shown in FIG. 9. In a case of FIG.1B, the gain control current Ic(T) having a temperature dependency isgenerated by adding the current I_(CNTO) having substantially notemperature dependency to the current I_(CNT1(T)) having a temperaturedependency, and then input to a current source 901 shown in FIG. 9. Again equation used for a variable gain circuit using FET that anapplicant provides in prior application specification (refer to the U.S.patent application Ser. No. 09/696,972, the entire contents of which areincorporated herein by reference) is expressed with the followingequation (14).

G _(MOS)={square root}{square root over (exp(−d·V _(c)))}  (14)

[0056] where d expresses a constant. The equation (14) is not a functionof VT because it is supposed that the current ID1 of a circuit as shownin FIG. 8 is Io*exp (−d*Vc) in the U.S. patent application Ser. No.09/696,972, the entire contents of which are incorporated herein byreference. However, when the current ID is generated by a circuit asshown in FIG. 9, the gain is expressed with the following equation (15)and is a function of V_(T). $\begin{matrix}\begin{matrix}{G_{MOS} = \sqrt{\exp \left( \frac{{- d_{1}} \cdot V_{C}}{n \cdot V_{T}} \right)}} \\{= {\exp \left( \frac{{- d_{1}} \cdot V_{C}}{2{n \cdot V_{T}}} \right)}}\end{matrix} & (15)\end{matrix}$

[0057] where d1 expresses a constant. Therefore, the present inventioncan be applied to a variable gain amplifier of the variable gain circuithaving FET which is disclosed in the Japanese Patent Application No.11-306798.

[0058] In other words, the temperature compensation can be performed bymultiplying an exponent of an equation (15) by a variable k (T)depending upon a temperature. (Explanation of FIGS. 7, 8 and 9)

[0059]FIG. 7 shows a variable gain amplifier 102 utilizing a weakinversion region of a MOS transistor. A gain control signal (first gaincontrol signal) Vc for controlling a gain of the variable gain amplifier102 from the outside is input to the gain control signal input terminal701. The first gain control signal Vc is converted to a second gaincontrol signal Vy by a control signal converter 702, and then issupplied to a variable gain amplifier fabricated by first differentialtransistor pair (M1, M2).

[0060] The variable gain amplifier is a circuit whose gain is controlledby the second gain control signal Vy, and comprises a first differentialtransistor pair of N type MOS transistors M1 and M2 which are operatedin the weak inversion region. A common source terminal of thetransistors M1, M2 is connected to a current source 703. An input signalcurrent I_(in) to be amplified is supplied from the current source 703to the differential transistor pair and an output terminal Iout is takenout from the drain terminal of the transistor M1. The currentI_(in)-I_(out) flowing through the drain terminal of the transistor M2is an unnecessary current, and flows to a power supply and so on. Thesecond gain control signal Vy output by control signal conversion 702 isa voltage signal, and input between the gate terminals of the transistorM1 and M2 constructing a differential transistor pair.

[0061]FIG. 8 shows a concrete circuit of gain control signal converter702 of FIG. 7. This gain control signal converter 702 has a seconddifferential transistor pair of N type MOS transistors MN10 and MN11that operate in the weak inversion region, and a current source 802 isconnected to a common terminal of the transistors MN10 and MN11. Adirect current Io is input to the second differential transistor pair bythe current source 802. The drain and gate terminals of the transistorMN10 are connected to each other and a current ID1=Io*exp (−b*Vc/VT) isinput into the drain terminal. The gate terminal of the transistor MN11is applied with a constant direct current level by a power supply V_(BB)and the drain terminal of the transistor MN11 is connected to a powersupply V_(DD) (not shown), for example.

[0062] The current source 801 for generating a current ID1 to besupplied to the gain control signal converter 702 of FIG. 8 is describedin conjunction with FIG. 9. In FIG. 9, N type MOS transistors MN20 andMN21 operate in the weak inversion region similarly to the transistorsMN10 and MN11.

[0063] There will now be described only about a point to be differentfrom FIG. 8 hereinafter. The current Io of the current source 802 ofFIG. 8 is generated by the voltage source V_(BB) and transistor MN21 ofFIG. 9. The gate terminal of the transistor MN21 is connected to thevoltage source V_(BB) and also to the gate terminal of the transistorMN20 and a gain controlled current source 901 (Ic=u/Vc) via a resistorR. Ic expresses a current. proportional to the voltage of the secondgain control signal Vc (a proportion coefficient is u). Since thecurrent IC is generated in easy using a voltage-to-current convertersuch as a differential circuit wherein a source degenerate resistor isconnected between source terminals, the detail description is omitted.

[0064] The source of the transistor MN20 is grounded, and its drainterminal is connected to an input terminal of a current mirror circuitfabricated by transistors MN20 and MP21 (gate and drain terminals of thetransistor MP20). The drain terminal of the transistor MP21 which is anoutput terminal of the current mirror circuit is connected to the drainand gate terminals of the transistor MN10. The currentID1=Io*exp(−b*Vc/VT) of the current source 801 of FIG. 8 is generated bya part of a circuit of FIG. 9. When a variable gain amplifier isfabricated using the circuit of FIG. 7, the ratio of an input current toan output current represents an exponential-law characteristic as isindicated in the equation (4).

[0065] (Second Embodiment)

[0066] A variable gain amplification circuit related to an embodiment ofthe present invention is suitable for a radio communication apparatus ofportable communication equipment using a direct conversion system. FIG.6 shows a configuration of a transceiver of a radio communicationapparatus of the direct conversion system. The present embodiment isdescribed based on a TDD (Time Division Duplex) system for performing achange of transmission and reception by time sharing, but the presentembodiment is not limited to this system.

[0067] On a transmitter side, the first and the second transmissionbaseband signals Ich (TX) and Qch (TX) orthogonal to each other areband-limited by a suitable filter in a baseband signal generator (TX-BB)601. These orthogonal baseband signals Ich (TX) and Qch (TX) areamplified, respectively, by baseband signal amplifiers 602 and 603 eachcomprising a variable gain amplifier related to the present embodiment.Then, the orthogonal transmission baseband signals Ich (TX) and Qch (TX)are input to an orthogonal modulator 607 comprising multipliers 604 and605 and an adder 606.

[0068] The orthogonal modulator 607 modulates, with the baseband signalsIch (TX) and Qch (TX), two orthogonal local oscillation signalsgenerated by dividing a local oscillation signal (frequency fLO11) of alocal oscillator 608 by a 90° phase shifter (90°-PS) 609. An unnecessarycomponent is removed from a modulated signal output from the orthogonalmodulator 607 (a RF (radio) signal) by a band passage filter 610 andinput to an power amplifier (PA) 611. In power amplifier 611, the RFsignal is adjusted to a suitable signal level by a RF stage variablegain amplifier provided on an input stage based on a control signal froma controller 624 and then is amplified to a predetermined power level.

[0069] The controller 624 is so constructed as to generate a controlsignal corresponding to a predetermined transmission power and areceived signal power. Concretely, the controller 624 comprises a tablewhich outputs control data corresponding to the predeterminedtransmission power and receiver signal power and a D/A converter whichconvert the control data to an analog control signal. The control signalfrom the controller 624 is supplied to the variable gain amplifiers 602,603, 611, 615, 621 and 622 to control the gains of the variable gainamplifiers. An amplified RF signal is emitted as a radio wave from anantenna (ANT) 613 via a transmit-receive switch (T/R) 612 or duplexer.

[0070] On a receiver side, a received RF signal from the antenna 613 isinput to a low-noise amplifier (LNA) 614 via the transmit-receive switch612. The received RF signal amplified by low-noise amplifier 614 isinput to an orthogonal demodulator (down converter) 618 comprising adivider and two multipliers 616 and 617 via a filter 615.

[0071] The orthogonal demodulator 618 frequency-converts the received RFsignal by multiplying the received RF signal and two orthogonal localoscillation signals, and outputs the orthogonal first and the secondbaseband signals Ich (RX) and Qch (RX). The two orthogonal localoscillation signals are generated by dividing a local oscillation signal(frequency fL010) of a local oscillator 619 by a 90° phase shifter(90°-PS) 620. These baseband signals Ich (RX) and Qch (RX) are amplifiedby baseband amplifiers 621 and 622 each comprising the variable gainamplification circuit related to the embodiment of the presentinvention. The amplified baseband signals Ich (RX) and Qch (RX) areinput to a baseband signal processor (RX-BB) 623 and demodulated by itto be reproduced in an original data signal. Generally, the gain on thereceiver is controlled by the low-noise amplifier 614 and the basebandamplifiers 621 and 622.

[0072] In a CDMA system developed in late years, when each power oftransmission signals from plural users, which is of the same frequency,varies widely, right communications cannot be realized. Therefore, atransmission power control over a wide range of more than 70 dB forexample is performed on a radio terminal side according to a distancefrom the terminal to a base station.

[0073] When the CDMA system uses radio equipment of the directconversion system as shown in FIG. 6, a variable gain range of a RFstage variable gain amplifier arranged in an input stage of the poweramplifier 611 for example is limited by an input-to-output isolation.For this reason, it is required to provide the baseband signalamplifiers 621 and 622 with a variable gain amplification function for atransmission power control.

[0074] It is desired that a lot of parts of a circuit are fabricated byMOS transistors to realize radio equipment with a low cost. The poweramplifier 611, low-noise amplifier 614, orthogonal modulator 607 andorthogonal demodulator 618 and the like which belong to an analog radiocircuit must be operated over a range from a low frequency to a highfrequency. Therefore, these devices had better be fabricated withbipolar transistors having a good frequency characteristic is desirable.

[0075] In contrast, since the baseband signal amplifiers 602, 603, 621and 622 treat a baseband signal of a relatively narrow band, theseamplifiers may be fabricated by MOS transistors inferior to the bipolartransistor in a frequency characteristic. Therefore, a variable gainamplification circuit realized by MOS transistors as being theembodiments of the present invention is suitable for radio equipmentproviding the baseband signal amplifiers 602, 603,, 621 and 622 with avariable gain control function.

[0076] According to the present invention, a temperature change of again characteristic of a variable gain amplifier using a MOS transistorcan be suppressed.

[0077] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A temperature compensation circuit performing atemperature compensation with respect to a gain characteristic of avariable gain amplifier fabricated by MOS transistors andgain-controlled by an external gain control signal, the circuitcomprising: a signal source configured to output a first signalcorresponding to a temperature change of an ambient temperature to apredetermined temperature; and a multiplier configured to multiply theexternal gain control signal and the first signal and to output a secondsignal proportional to the temperature change to the variable gainamplifier.
 2. A circuit according to claim 1, which includes anamplifier configured to amplify the external gain control signal andconvert it into a gain control current to be supplied to the multiplier.3. A circuit according to claim 1, which includes a start-up circuitconfigured to drive the signal source.
 4. A circuit according to claim1, wherein the multiplier comprises a first current source generating atail current Io having substantially no temperature dependency, a secondcurrent source generating a tail current Io (1+ΔT/To) having atemperature dependency, a third current source generating a gain controlcurrent Ic before the temperature compensation, and differential MOStransistor pairs connected to the first and second current sources,respectively, and outputs a current represented by Ic(1+ΔT/To) as thesecond signal.
 5. A circuit according to claim 4, wherein thedifferential MOS transistor pairs are operated in the weak inversionregion.
 6. A circuit according to claim 1, wherein the signal sourcecomprises a first MOS transistor whose source terminal is grounded, asecond MOS transistor having gate and drain terminals connected to agate of the first MOS transistor, and a resistor through which a sourceterminal of the second MOS transistor is grounded, and substantiallyidentical currents flow through the drain terminals of the first andsecond MOS transistors, and the first and second MOS transistorsoperates in a weak inversion region.
 7. A circuit according to claim 6,wherein the signal source includes a third MOS transistor connected tothe gate and drain terminals of the second MOS transistor and a start-upcircuit connected to a gate of the third MOS transistor to drive thesignal source.
 8. A circuit according to claim 6, which includes acurrent mirror circuit arranged between the signal source and themultiplier, the current mirror circuit comprising MOS transistors and aconstant current source having substantially no temperature dependency.9. A circuit according to claim 1, wherein the multiplier includes aconstant current source having substantially no temperature dependency,the multiplier equalizing substantially a ratio between an output of theconstant current source and the external gain control signal and a ratiobetween the first signal and the second signal.
 10. A circuit accordingto claim 9, wherein the constant current source includes a first currentsource that outputs a current proportional to the thermal voltage, asecond current source that outputs a current proportional to a thresholdvoltage of the MOS transistors, and an adder configured to add thecurrent of the first current source and the current of the secondcurrent source to generate a current having substantially no thermaldependency.
 11. A circuit according to claim 10, wherein the firstcurrent source comprises a first MOS transistor whose source terminal isgrounded, a second MOS transistor having gate and drain terminalsconnected to a gate of the first MOS transistor, and a resistor throughwhich a source terminal of the second MOS transistor is grounded, andsubstantially identical currents flow through the drain terminals of thefirst and second MOS transistors, and the first and second MOStransistors operates in a weak inversion region.
 12. A circuit accordingto claim 11, wherein the second current source comprises a third MOStransistor whose source terminal is grounded, a resistor through which agate of the third MOS transistor is grounded, and a fourth MOStransistor having a gate connected to a drain of the third MOStransistor and a source grounded via the resistor.
 13. A circuitaccording to claim 12, wherein the third MOS transistor is operated in aweak inversion region.
 14. A temperature compensation circuit performinga temperature compensation with respect to a gain characteristic of avariable gain amplifier, using an external gain control signal, theapparatus comprising: a signal source configured to output a firstsignal corresponding to a temperature change of an ambient temperatureto a predetermined temperature; a multiplier configured to multiply theexternal gain control signal and the first signal and output a secondsignal proportional to the temperature change and the external gaincontrol signal; a differential amplifier configured to be supplied withthe external gain control signal and output a third signal (I_(CNTO))having substantially no temperature dependency, the differentialamplifier including a source regenerate resistor; and an adderconfigured to add the second signal and the third signal to output afourth signal to the variable gain amplifier.
 15. A variable gainamplification circuit, comprising: a variable gain amplifier -fabricatedby MOS transistors and gain-controlled by an external gain controlsignal; and a temperature compensation circuit configured to perform atemperature compensation with respect to the external gain controlsignal, the temperature compensation circuit including a signal sourceconfigured to output a first signal corresponding to a temperaturechange of an ambient temperature to a predetermined temperature, and amultiplier configured to multiply the external gain control signal andthe first signal and output a second signal proportional to thetemperature change and the external gain control signal to the variablegain control amplifier.
 16. A circuit according to claim 15, wherein thesignal source comprises a first MOS transistor whose source terminal isgrounded, a second MOS transistor having gate and drain terminalsconnected to a gate of the first MOS transistor, and a resistor throughwhich a source terminal of the second MOS transistor is grounded, andsubstantially identical currents flow through the drain terminals of thefirst and second MOS transistors, and the first and second MOStransistors operates in a weak inversion region.
 17. A circuit accordingto claim 16, wherein includes a current mirror circuit arranged betweenthe signal source and the multiplier, the current mirror circuitcomprising MOS transistors and a constant current source havingsubstantially no temperature dependency.
 18. A circuit according toclaim 15, wherein the multiplier includes a constant current sourcehaving substantially no temperature dependency, the multiplierequalizing substantially a ratio between an output (Io) of the constantcurrent source and the external gain control signal and a ratio betweenthe first signal and the second signal.
 19. A circuit according to claim18, wherein the constant current source includes a first current sourcethat outputs a current proportional to the thermal voltage, a secondcurrent source that outputs a current proportional to a thresholdvoltage of the MOS transistors, and an adder configured to add thecurrent of the first current source and the current of the secondcurrent source to generate a current having substantially no thermaldependency.
 20. A radio communication apparatus comprising: atransmitter including a baseband signal generator to generate a basebandsignal, a baseband signal amplifier to amplify the baseband signal, anorthogonal modulator to orthogonal-modulate the baseband signalamplified by the amplifier, and a power amplifier to amplify a modulatedsignal of the orthogonal modulator; and a receiver including a low-noiseamplifier to amplify a received signal, an orthogonal demodulator toorthogonal-demodulate the received signal amplified by the amplifier, abaseband signal amplifier to amplify a demodulated signal of theorthogonal demodulator, and a baseband signal processor to process thebaseband signal obtained by the baseband signal amplifier of thereceiver, each of the baseband signal amplifiers and power amplifier ofthe transmitter being configured by the variable gain amplifier circuitaccording to claim 15, and each of the baseband amplifiers and low-noiseamplifier of the receiver being configured by the variable gainamplifier circuit.
 21. A temperature compensation method of performing atemperature compensation with respect to a gain characteristic of avariable gain amplifier fabricated by MOS transistors andgain-controlled by an external gain control signal, the methodcomprising: generating a first signal corresponding to a temperaturechange of an ambient temperature to a predetermined temperature; andmultiplying the external gain control signal and the first signal andoutput a second signal proportional to the temperature change and thefirst signal to the variable gain amplifier to perform the temperaturecompensation with respect to the variable gain amplifier.